The present invention relates to an improvement of a plastic encapsulated semiconductor device and a method for manufacturing the same and, more particularly, to an improvement of an electrical insulating layer on a heat sink of the semiconductor device and a method for manufacturing the same.
Plastic encapsulated semiconductor devices such as power transistors, each of which has a heat sink, have long been commercially available. Plastic encapsulation in the manufacturing process of these semiconductor devices is performed as follows. FIG. 1 is a sectional view of a semi-finished semiconductor device to be plastic encapsulated and a device to plastic encapsulate it. Referring to FIG. 1, a lead frame 1 for the semiconductor device has a mount portion 3 and a lead portion 5. A semiconductor chip (pellet) 7 is mounted on a mount surface 3a of the mount portion 3. The mount portion 3 also serves as a heat sink for dissipating heat generated from the semiconductor chip 7. The surface of the mount portion 3 which is opposite to the mount surface 3a thereof serves as a heat dissipation surface 3b.
The lead frame 1 is fixed between a preheated upper mold 9 and a preheated lower mold 11. Both the upper and lower molds are substantially bilaterally symmetrical. A plurality of upper and lower molds 9, 11 and a plurality of lead frames 1 extend in such a way as to be perpendicular to the sheet of the drawing, so that a plurality of semiconductor devices may be simultaneously plastic-encapsulated. A resin for encapsulation is injected from a runner 13. The injected resin flows in the right and left directions and is used to fill cavities 13a and 13b, through gates 15. In this case, the resin flows at the identical speed in the cavity 13a on the side of the mount surface 3a of the mount portion 3 and the cavity 13b on the side of the heat dissipation surface 3b thereof. However, in general, cavity 13b is narrower than cavity 13a. This is because the thickness of the resin on the heat dissipation surface 3b of the mount portion 3 must be made as thin as possible to improve the heat dissipation effect. Therefore, cavity 13a has a different resistance to the flow of the resin than that of cavity 13b. The resistance to the flow of the resin in cavity 13a is lower than that in cavity 13b. For this reason, the flow velocity of the resin in cavity 13b is decreased. The resin may occasionally be solidified in cavity 13b. However, since the resistance to the flow of the resin in cavity 13a is low, the resin may smoothly flow therein. As a result, after cavity 13a is filled with the resin, it flows into cavity 13b. For this reason, cavity 13b is filled with resin, which flows in the direction indicated by arrow 15b; and is also filled with resin flowing in the direction shown by arrow 15c, after cavity 13a is filled. However, according to this filling method, bubbles 17 are formed at portions wherein the resin flowing in direction (15b) meets the resin flowing in direction (15c). In extreme cases, non-filled portions remain. So pinholes or holes may be formed in the package of a semiconductor device encapsulated by the above-described method. Furthermore, the package has a non-uniform thickness. This becomes disadvantageous in protecting the device from humidity and an impurity and in electrical insulation of the mount portion 3 of the lead frame 1. In addition to these disadvantages, the yield of the semiconductor devices is decreased. To solve these problems, cavity 13b may be so enlarged as to have the same size as cavity 13a. However, the resin layer on the heat dissipation surface 3b of the mount portion 3 of the lead frame 1 must be thin, in favor of effective heat dissipation. Therefore, according to the conventional method of plastic encapsulation of the semiconductor device, contradictory problems arise, which are difficult to solve, in that the cavity 13b must be narrow to improve the heat dissipation effect; a narrow cavity 13b is difficult to completely fill with resin, without forming bubbles; and a high manufacturing yield must be maintained. According to the conventional semiconductor device manufacturing method, it is difficult to decrease the thickness of the resin layer on the heat dissipation surface, to improve the heat dissipation effect and obtain a package of uniform thickness, without the appearance of pinholes and holes.
To solve the above problems, an improved plastic encapsulation method has been proposed. As shown in FIG. 3, a dam-like portion 19 is formed in a cavity 21a of the mold which is located on the side of a semiconductor chip 7. Therefore, cavity 21a and cavity 21b have the same resistance to the flows of the resin used for encapsulation, as indicated by the arrows. However, according to this method, the design of the package of the semiconductor device must be modified, resulting in a design limitation. Furthermore, since a special mold must be formed, the conventional mold cannot be used, resulting in inconvenience.